NO.sub.x sensors for measuring the concentration of nitrogen oxides (NO.sub.x) contained in gases to be analyzed (hereinafter called "a measurement gas") are disclosed, for example, in European Patent Application Laid-Open No. 0678740A1 and SAE Paper No. 960334, pp. 137-142, 1996. Such conventional NO.sub.x sensors are composed of oxygen ion conductive solid electrolyte layers that form a first measurement space and a second measurement space. The first measurement space communicates with a measurement gas via a first diffusion controlling layer, and the second measurement space communicates with the first measurement space via a second diffusion controlling layer. Furthermore, the solid electrolyte layer of the first measurement space is sandwiched between porous electrodes so as to form a first oxygen pumping cell and an oxygen concentration measuring cell. Also, the solid electrolyte layer of the second measurement space is sandwiched between porous electrodes so as to form a second oxygen pumping cell.
In the thus configured NO.sub.x sensor, the first oxygen pumping cell is energized so that an output voltage from the oxygen concentration measuring cell achieves a predetermined value, to thereby pump out oxygen from the first measurement space and thus control the concentration of oxygen in the first measurement space to a constant level. At the same time, a constant voltage is applied to the second oxygen pumping cell to thereby pump out oxygen from the second measurement space. As a result, the NO.sub.x concentration of the measurement gas can be obtained by measuring the current flowing through the second oxygen pumping cell (hereinafter referred to as "second pumping current").
A measurement gas, e.g., exhaust from an internal combustion engine or the like, contains gas components other than NO.sub.x, such as oxygen, carbon monoxide and carbon dioxide. Thus, in the aforementioned NO.sub.x sensor, current (hereinafter referred to as "first pumping current") is first applied to the first oxygen pumping cell to thereby pump out most of the oxygen from a measurement gas contained in the first measurement space. Then, in the second measurement space into which the oxygen-removed measurement gas flows, NO.sub.x contained in the measurement gas is decomposed into nitrogen and oxygen by means of the catalyzing function of the second oxygen pumping cell. The thus generated oxygen is then pumped out from the second measurement space. Thus, the NO.sub.x concentration of the measurement gas can be obtained by measuring the second pumping current without being affected by other gas components contained in the measurement gas.
In order to accurately measure the NO.sub.x concentration using the above described NO.sub.x sensor, the NO.sub.x sensor must be heated to a predetermined activation temperature (for example, 800.degree. C. or higher) so as to activate the pumping cells. Accordingly, the NO.sub.x sensor is provided with a heater, and current applied to the heater is controlled so as to control the temperature of the NO.sub.x sensor at a predetermined level.
However, in a conventional NO.sub.x sensor, the NO.sub.x concentration obtained from the second pumping current must be appropriately compensated in order to provide an accurate measurement. This requires a complex signal processing system, with a resulting increase in the cost of the sensing apparatus. The above noted problems are described in detail below.
According to the design concept of a conventional NO.sub.x sensor, oxygen is pumped out from the first measurement space using the first oxygen pumping cell so as to control the measurement gas contained in the first measurement space at a very low oxygen concentration level. As a result, the measurement gas flowing into the second measurement space contains substantially NO.sub.x only. By decomposing the measurement gas into nitrogen and oxygen by means of the catalyzing function of the second oxygen pumping cell, the NO.sub.x concentration can be obtained from the second pumping current flowing through the second oxygen pumping cell.
However, in actuality, if the first oxygen pumping cell is controlled so that the concentration of oxygen contained in the first measurement space becomes substantially zero (theoretically, a partial pressure of about 10.sup.-9 atm), the NO.sub.x concentration cannot be obtained from the second pumping current. Thus, in order to measure the NO.sub.x concentration at a relatively high detection sensitivity using a conventional NO.sub.x sensor, the first oxygen pumping cell must be controlled such that the concentration of oxygen contained in the first measurement space becomes as low as about 1000 ppm.
A reason has been proposed as to why the NO.sub.x concentration cannot be obtained at a good detection sensitivity when the concentration of oxygen in the first measurement space is controlled to be substantially zero. Namely, as a result of controlling the first pumping current, the NO.sub.x component of a measurement gas contained in the first measurement space is decomposed. Consequently, the measurement gas flowing into the second measurement space does not contain NO.sub.x in an amount that is the same as that contained in the actual measurement gas to be analyzed.
Accordingly, when the NO.sub.x concentration is obtained from the second pumping current while the first oxygen pumping cell is controlled such that the concentration of oxygen contained in the first measurement space becomes as low as about 1000 ppm, the second pumping current varies in accordance with the NO.sub.x concentration of the measurement gas. However, the second pumping current is also affected by the oxygen concentration of the measurement gas. This is because the measurement gas flowing from the first measurement space into the second measurement space contains not only NO.sub.x but also oxygen. As a result, conventional NO.sub.x sensors fail to indicate the actual NO.sub.x concentration. This is because the NO.sub.x concentration thus obtained is affected by the oxygen concentration of the measurement gas present around the NO.sub.x sensor.
This problem can be solved, for example, by measuring the oxygen concentration of the measurement gas present around the NO.sub.x sensor based on the first pumping current and compensating the NO.sub.x concentration thus obtained in accordance with the measured oxygen concentration. That is, the first pumping current is controlled so that the concentration of oxygen contained in the first measurement space is maintained at a constant level. Also, the value of the first pumping current is proportional to the oxygen concentration of the measurement gas present around the NO.sub.x sensor. Thus, by obtaining the oxygen concentration of the ambient atmosphere from the first pumping current and thereby compensating the measured NO.sub.x concentration, an accurate NO.sub.x concentration can be obtained.
However, in order to compensate the measured NO.sub.x concentration by obtaining the oxygen concentration of the measurement gas as described above, additional compensation means are required which leads to an increase in the cost of the sensing apparatus. The present invention has been accomplished in view of the above described problems of the prior art.